Two-index selectively coated Fresnel

ABSTRACT

An optical structure includes a first refractive layer, a Fresnel surface, a dichroic reflective coating, a third refractive layer, and a replica layer. The first refractive layer has a first refractive index. The Fresnel surface is formed in a second refractive layer having a second refractive index. The Fresnel surface includes active surfaces and draft surfaces. The dichroic reflective coating is selectively disposed on the active surfaces. The replica layer is disposed between the Fresnel surface and the third refractive layer. The replica layer has the second refractive index and the second refractive index is higher than the first refractive index.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. non-provisional patent applicationsentitled, “Multilayer Dichroic Phase Matching for Fresnel Optics” and“Phase-Shift Matched Fresnel Layers,” filed the same day.

TECHNICAL FIELD

This disclosure relates generally to optics, and in particular totwo-index Fresnel optics.

BACKGROUND INFORMATION

Lenses and other optical elements are ubiquitous in both consumer andcommercial products such as cameras, displays, and sensors. Fresnellenses were famously developed in the early 1800s and originallydeployed in lighthouses to increase the distance that the light from thelighthouse could be viewed by ships. Using Fresnel lenses can beadvantageous in that they are thinner (and often lighter) thanconventional lenses with similar optical power. In some contexts,Fresnel optical elements are used to provide lensing for particularportions of the light spectrum. Yet, conventional designs for Fresneloptical elements that provide lensing for certain light spectrumspresent optical integrity challenges.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIGS. 1A and 1B illustrate example Fresnel optical elements redirectingincoming light, in accordance with embodiments of the disclosure.

FIGS. 2A and 2B illustrate a portion of a Fresnel optical element havinga Fresnel surface including active surfaces and draft surfaces, inaccordance with an embodiment of the disclosure.

FIG. 3 illustrates a Fresnel reflector having selectively coated activesurfaces.

FIGS. 4A and 4B illustrates an example Fresnel surface having a coatingon the active surfaces that is index-matched to a refractive material,in accordance with an embodiment of the disclosure.

FIG. 5 illustrates an example Fresnel surface having a first coatingdeposited on the active surfaces that imparts a same phase jump as asecond coating disposed on the draft surfaces, in accordance with anembodiment of the disclosure.

FIG. 6 illustrates an example two-index Fresnel reflector withselectively coated active surfaces, in accordance with an embodiment ofthe disclosure.

FIGS. 7A-7E illustrate an example process of fabricating a two-indexFresnel reflector, in accordance with an embodiment of the disclosure.

FIGS. 8A-8E illustrate another example process of fabricating atwo-index Fresnel reflector, in accordance with an embodiment of thedisclosure.

FIG. 9 illustrates an example head mounted display (HMD) that mayinclude a Fresnel surface for directing infrared light reflected off aneye to a camera, in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Embodiments of a Fresnel optical elements and processes of fabricatingFresnel optical elements are described herein. In the followingdescription, numerous specific details are set forth to provide athorough understanding of the embodiments. One skilled in the relevantart will recognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

The Fresnel optical elements and processes of fabricating Fresneloptical elements are described in this disclosure. The Fresnel opticalelements of this disclosure may be used in a head mounted display (HMD)to direct infrared light reflecting off an eye of a wearer of the HMD toa camera while also allowing scene light to propagate to the eye(s) ofthe wearer. Embodiments of Fresnel optical elements of the disclosureinclude active surfaces that are selectively coated with a “hot mirror”layer (reflecting infrared light and passing visible light) to directinfrared light to a camera while also passing scene light for viewing bythe wearer of the HMD. Of course, the embodiments of the disclosure maybe used in other contexts, in addition to HMDs.

Prior Fresnel optical elements included Fresnel surfaces withselectively coated active surfaces. However, the selective coating onthe active surface may cause undesirable optical artifacts fromsee-through scene light having different optical path lengths due to thescene light propagating through the hot mirror coating on the activesurfaces and the uncoated draft surfaces. In particular, a diffractioneffect may be induced by the periodic phase jump and periodictransmissivity variation and stray light issues (e.g. ghost images) mayarise from multi-reflection in the Fresnel structure. Since the hotmirror coating introduces a different refractive index, the scene lightmay encounter different optical path lengths when propagating throughthe coated active surface and the uncoated draft surface and undesirableconstructive or destructive interference of the scene light may occur asa result.

Fresnel surfaces having active surfaces that are selectively coated witha “hot mirror” are disclosed along with techniques to reduce oreliminate a phase differential of scene light propagating through theFresnel surface. In one embodiment, the hot mirror coating on the activesurface is index-matched to a refractive material that the Fresnelsurface is formed in so that scene light (in the visible spectrum)encounters the same or similar index of refraction whether it propagatesthrough the draft surfaces or the active surfaces of the Fresnelsurface. In one embodiment, the draft surface is coated with a secondcoating with a refractive index that gives scene light passing throughthe Fresnel surface the same optical path length whether the scene lightpropagates through the draft surface of the active surface. In this way,scene light propagating through the dichroic reflective coatingexperiences the same phase shift as the scene light propagating throughthe second coating. The Fresnel surfaces along with the selectivecoating on the active surfaces may be “immersed” in a material havingthe same refractive index as the material that the Fresnel surface isformed in to keep the optical path length the same or similar for scenelight.

In an embodiment of the disclosure, a two-index optical structureincludes a Fresnel surface with selectively coated active surfaces. TheFresnel surface and a replica layer that is a negative of the Fresnelsurface is formed in a higher-index material with a higher refractiveindex than a first refractive layer that the Fresnel surface is disposedon. The higher-index material may be matched to an average refractiveindex of the coating on the active surfaces. The thickness of thehigher-index material may be limited to the extent that only a thicknessof the higher-index material necessary to form the Fresnel surface isutilized. Reducing the thickness of the higher-index material mayimprove the viewing of images from scene light passing through thetwo-index optical structure.

These and other embodiments are described in detail below in associationwith FIGS. 1A-9.

FIG. 1A illustrates a cross-section of an on-axis Fresnel reflector 101receiving incoming light 111 (illustrated with dashed lines) andfocusing the light (in reflection) to a point 120. The reflected lightis illustrated as solid lines, in FIGS. 1A and 1B. Fresnel reflectorscan be utilized to direct and focus light, in reflection.

FIG. 1B illustrates an optical combiner 100 that includes a Fresneloptical element that directs reflected infrared light 151 to camera 147while allowing scene light 199 to propagate through optical combiner 100to eye 102, in accordance with an embodiment of the disclosure.Reflected infrared light 151 is reflected from eye 102 and a Fresneloptical reflector included in optical combiner 100 directs and focusesthe infrared light to camera 147. The Fresnel optical reflector includedin optical combiner 100 may be a one dimensional prism array or anoff-axis Fresnel lensing shape to direct reflected infrared light 151 tocamera 147. Optics 143 may include one or more lenses to assist infocusing the infrared light for camera 147. Camera 147 may be a CMOSimage sensor that includes a filter that passes infrared light whilerejecting visible light (e.g. wavelength of 380 nm-730 nm). Camera 147may be included in an HMD such as HMD 900 that will be discussed in moredetail in connection with the description of FIG. 9. Eye 102 includespupil 103 and may be illuminated by infrared light emitted from a diodeinclude in an HMD. Camera 147 may be configured to image eye 102 todetermine a position of pupil 103 for eye-tracking purposes.

FIG. 2A illustrates a side view of an example Fresnel surface 200 formedin a refractive material 201. The illustrated Fresnel optical surface200 is rotationally symmetric around a central axis 299, although notall Fresnel optical elements are rotationally symmetric. FIG. 2Aillustrates that the Fresnel features of the Fresnel surface mayincrease in size as a distance of the Fresnel feature from the centralaxis 299 increases.

FIG. 2B illustrates a zoomed-in view of view 293 of FIG. 2A. FIG. 2Bincludes Fresnel features 210A and 210B. Fresnel features 210 eachinclude a draft surface 213 (dashed lines) and an active surface 215(solid line). Active surfaces 215 provide the optical power for Fresnelsurface 200. Although only two Fresnel features are illustrated asexamples throughout the disclosure, those skilled in the art understandthat each Fresnel optical element may have many more Fresnel features210 and that the techniques disclosed herein may be applied to all ofthe Fresnel features in a Fresnel optical element.

FIG. 3 illustrates a side view of a portion of a Fresnel reflectorhaving active surfaces selectively coated with a reflective material.FIG. 3 illustrates a Fresnel surface 300 formed in a refractive material301. Fresnel surface 300 includes a first Fresnel feature that includesdraft surface 313A and an active surface 315A. Fresnel surface 300 alsoincludes a second Fresnel feature that includes a draft surface 313B andan active surface 315B. The active surfaces 315A and 315B areselectively coated with coating segments 325A and 325B, respectively.

Coating 325 may be a hot mirror coating, for example. The hot mirrorcoating may reflect infrared light while passing substantially all lightin the visible spectrum. Fresnel surface 300 may be configured to directand focus infrared light reflected off an eye of a wearer of an HMD to acamera while passing scene light (visible light) to an eye of a wearerof an HMD. However, a scene light image 399 may be out of phase becausecoating 325 introduces a difference in optical path lengths taken byscene light propagating along optical paths 351 and 352 due to thecoating 325 having a different refractive index as refractive material301. To illustrate, portions 391A and 391B of scene light image 399 areout of phase with portions 392A and 392B of scene light image 399 inFIG. 3. The out of phase nature of scene light image 399 may causeundesirable optical artifacts (e.g. rainbow effect, ghost images, andother stray light issues) that are noticeable by a wearer of a HMD thatis viewing scene light through an optical combiner that includes Fresnelsurface 300.

FIG. 4A illustrates an optical structure 450 including a Fresnel surface400 formed in a refractive material 401, in accordance with anembodiment of the disclosure. Refractive material 401 may include any ofpoly-methyl-methacrylate (PMMA), polycarbonate, cyclic-olefin polymer(e.g. Zeonex™), styrene, or polyester (e.g. OKP-4™). Fresnel surface 400includes active surfaces 415 and draft surfaces 413. A coating 433 isselectively disposed on the active surfaces 415, but not the draftsurfaces 413. In FIG. 4, coating segment 433A is disposed on activesurface 415A and coating segment 433B is disposed on active surface415B. Coating 433 may be a dichroic coating. In one embodiment, coating433 is a multilayer hot mirror coating that reflects near-infrared lightwhile passing visible light. Coating 433 may be a multilayer coatinghaving three or more layers.

FIG. 4B illustrates an example coating 433 having layers 443, 444, 445,and 446 with refractive indexes n1, n2, n3, and n4, respectively. Theaverage refractive index of coating 433 is substantially the same as therefractive index of refractive material 401. Coating 433 may have morethan four layers in some embodiments and the average refractive index ofthose layers would be substantially the same as the refractive index ofrefractive material 401. In one embodiment, layer 443 is made of thesame material as layer 445 and layer 444 is made of the same material aslayer 446. The thicknesses of layers 443, 444, 445, and 446 may be tuneto constructively and/or destructively interfere light of particularwavelengths in order to impart dichroic characteristics to coating 433.

In one embodiment, coating 433 includes at least one layer of niobiumpentoxide (Nb₂O₅) and at least one layer of silicon dioxide (SiO₂). Inone embodiment, coating 433 includes at least one layer of titaniumdioxide (TiO₂) and at least one layer of silicon dioxide. In oneembodiment, coating 433 includes at least one layer of silicon nitride(Si₃N₄) and at least one layer of magnesium fluoride (MgF₂). Coating 433may include other layers of dielectric material to achieve the desiredindex-matching coating design.

Referring again to FIG. 4A, coating 433 and the draft surfaces 413 areimmersed in a transparent material having a same refractive index as therefractive material 401. The transparent material is illustrated as thesame refractive material 401, in FIG. 4A. In one embodiment, refractivematerial 401 has a refractive index greater than 1.65. In oneembodiment, the refractive index of refractive material 401 is greaterthan 1.7. In one embodiment, the refractive index of refractive material401 is greater than 1.8. The refractive index of refractive material 401may be approximately 1.5, in some embodiments.

With coating 433 index-matched to refractive material 401 and Fresnelsurface 400, and with coating 433 immersed in refractive material 401,scene light propagating along optical paths 452A and 452B (throughcoating segments 433A/433B and active surfaces 415A/415B) encounters thesame or substantially the same refractive index as it propagates throughoptical structure 450. Similarly, scene light propagating along opticalpaths 451A and 451B (through draft surfaces 413A and 413B) alsoencounters the same refractive index (the refractive index of material401) that scene light propagating along optical paths 452A and 452Bdoes. Therefore, scene light image 499 is in phase as portions 491A and491B of scene light image 499 are in phase with portions 492A and 492Bof scene light image 499 in FIG. 4A.

Designers of optical structure 450 may select a refractive material 401having a high index of refraction (e.g. greater than 1.65) since coating433 may have a refractive index greater than 1.65. The thicknesses oflayers of coating 433 may then be tuned so that the average refractiveindex of coating 433 is the same as refractive material 401. Notably,the thicknesses of the layers of coating 433 must still be designed toachieve the desired dichroic attributes (e.g. hot mirror), in someembodiments. Designers of optical structure 450 may also select acoating 433 and then select a refractive material 401 having the same orsubstantially the same refractive index as the average refractive indexof coating 433. In one embodiment, the coating 433 is designed to havean average refractive index of approximately 1.5 and the refractivematerial 401 has a refractive index that matches.

FIG. 5 illustrates an optical structure 550 including a Fresnel surface500 formed in a refractive material 501, in accordance with anembodiment of the disclosure. Refractive material 501 may include any ofpoly-methyl-methacrylate (PMMA), polycarbonate, cyclic-olefin polymer(e.g. Zeonex™), styrene, or polyester (e.g. OKP-4™). Fresnel surface 500includes active surfaces 515 and draft surfaces. A coating 563 isselectively disposed on the active surfaces 515 and coating 561 isselectively disposed on the draft surfaces. In FIG. 5, coating segment563A is disposed on active surface 515A, coating segment 563B isdisposed on active surface 515B, and coating segments 561A and 561B aredisposed on the draft surfaces. Scene light (e.g. 551 and 552)propagating through the layer 563 experiences the same phase shift asthe scene light propagating through the layer 561.

In one embodiment, coating 563 is a multilayer hot mirror coating thatreflects near-infrared light while passing visible light. Coating 563may be a dichroic coating. Coating 563 may be a multilayer coatinghaving three or more layers. Coating 563 may include the embodiments ofcoating 433 described in connection with FIGS. 4A and 4B. Coating 561may be a multilayer coating or a single layer coating that passes bothvisible light and non-visible light (including infrared light).

A first average refractive index of the coating 563 is substantially thesame as a second average refractive index of coating 561, in oneembodiment. The thickness of coating 563 may be substantially the sameas a thickness of coating 561 from the perspective of incident scenelight so that the scene light has a same optical path length whenpropagating along optical path 551 or 552.

With coating 561 having an average refractive index substantially thesame as coating 563, the phase change of incident scene light that isimparted by coating 563 on active surfaces 515 is substantially the sameand coating 561 on the draft surfaces. And, since Fresnel surface 500and coatings 561/563 are immersed in refractive material 501, scenelight propagating along optical paths 551 and 552 have the same opticallength as it propagates through optical structure 550. Therefore, scenelight image 599 is in phase as portions 591A and 591B of scene lightimage 599 are in phase with portions 592A and 592B of scene light image599 in FIG. 5.

Designers of optical structure 550 may select a coating 561 to have thesame average refractive index of coating 563. Or designers of opticalstructure 550 may tune coating 563 to have the same average refractiveindex of coating 561. In one embodiment, coating 561 has an averagerefractive index that is different from an average refractive index ofcoating 563 while both coatings 561 and 563 impart the same phase shiftto incident scene light.

FIG. 6 illustrates an example two-index Fresnel reflector withselectively coated active surfaces, in accordance with an embodiment ofthe disclosure. FIG. 6 includes an optical structure 650 including afirst refractive layer 606 including a first refractive material 601having a first refractive index. Optical structure 650 also includes aFresnel surface 600 formed in a second refractive layer 607 including asecond refractive material 602 with a second refractive index differentthan the first refractive index. Second refractive material 602 may havea higher index of refraction than first refractive material 601. Fresnelsurface 600 includes active surfaces 615 and draft surfaces 613. Acoating 633 is selectively disposed on the active surfaces 615, but notthe draft surfaces 613. In FIG. 6, coating segment 633A is disposed onactive surface 615A and coating segment 633B is disposed on activesurface 615B. In one embodiment, coating 633 is a multilayer hot mirrorcoating that reflects near-infrared light while passing visible light.Coating 633 may include the properties of coating 433 described inconnection with FIGS. 4A and 4B.

A replica layer 608 is disposed between Fresnel surface 600 and a thirdrefractive layer 609, in FIG. 6. Replica layer 608 is a negative of theFresnel surface 600 and “immerses” the draft surfaces 613 and coating633 in the same refractive material 602 having the second refractiveindex, in the illustrated embodiment. In one embodiment, replica layer608 is a rigid element that is bonded to draft surfaces 613 and coating633 with a bonding material having the second refractive index. Thethird refractive layer 609 is made of the first refractive material 601having the first refractive index, in the illustrated embodiment. In oneembodiment (not illustrated), replica layer 608 is a cured material thatwas poured or injected into a gap between third refractive layer 609 andFresnel surface 600 and allowed to cure. The cured material has thesecond refractive index.

The optical structure 650 is similar to optical structure 450 in that anaverage refractive index of coating 633 is substantially the same as therefractive index of the material 602 that Fresnel surface 600 is formedin. The two-index design of optical structure 650 reduces the thicknessof a higher index material (e.g. 602) used to form Fresnel surface 600,which may reduce a “haze” effect when compared with Fresnel structuresthat are formed in thicker high-index refractive materials. Inparticular, the thickness of the higher-index material 602 may belimited to the thickness necessary to form the Fresnel surface 600 insecond refractive layer 607 and immerse the Fresnel surface 600 withreplica layer 608. Reducing the thickness of the higher-index materialmay improve the viewing of images from scene light passing through thetwo-index optical structure. First refractive layer 606 and thirdrefractive layer 609 may be included in optical structure 650 to improvethe structural rigidity and/or facilitate the manufacturability ofoptical structure 650 without inducing non-uniform phase changes inincident scene light propagating through optical structure 650.

FIGS. 7A-7E illustrates an example process of fabricating a two-indexFresnel reflector using a bonded replica layer, in accordance with anembodiment of the disclosure. In FIG. 7A, a first refractive layer 606having a first refractive index is provided.

In FIG. 7B, a Fresnel surface 600 including active surfaces 615 anddraft surfaces 613 is formed in a second refractive layer 607. Secondrefractive layer 607 may be bonded to first refractive layer 606 and thesecond refractive layer 607 may be diamond-turned to form theillustrated Fresnel surface 600. Bonding the second refractive layer 607to the first refractive layer 606 may give the optical structure of FIG.7B the rigidity it needs to undergo a diamond turning process step whilestill limiting the higher index material 602 to the thickness requiredto support the formation of Fresnel surface 600.

In FIG. 7C, coating 633 is selectively forming on the active surfaces615 of the Fresnel surface.

In FIG. 7D replica layer 608 is bonded to the coating 633 disposed onthe active surfaces 615 of the Fresnel surface. The replica layer 608may be bonded with an index-matching bonding material having the secondrefractive index (same refractive index as material 602). In theillustrated embodiment, replica layer 608 is bonded to the thirdrefractive layer 609. The negative image of Fresnel surface 600 may beformed on the replica layer 608 by a subtractive diamond-turning processwhile the second refractive material 602 is bonded to first refractivematerial 601 of third refractive layer 609.

FIG. 7E illustrates the resulting optical structure 750 after thereplica layer 608 is bonded to second refractive layer 607 and thecoating 633 that is disposed on active surfaces 615.

FIGS. 8A-8E illustrates an example process of fabricating a two-indexFresnel reflector using a cured replica layer, in accordance with anembodiment of the disclosure.

FIGS. 8A-8C illustrate the same fabrication techniques as FIGS. 7A-7C.In FIG. 8D, third refractive layer 609 is secured over the Fresnelsurface 600. In the illustrated embodiment, the third refractive layer609 has the first refractive index because it is made of the samematerial 601 as the first refractive layer. In some embodiments, thethird refractive layer 609 is a different material than material 601.

In FIG. 8E, replica gaps between the third refractive layer 609 and theFresnel surface are filled with a curable material 802 having the secondindex of refraction (same as material 602). The curable material 802 isthen cured, which may adhere third refractive layer 609 to Fresnelsurface 600. Curable material 802 may be adhered to coating 633, thedraft surfaces 613, and the third refractive layer 609.

FIG. 9 illustrates an example HMD 900 that may include a Fresnel surfacefor directing infrared light reflected off an eye to a camera, inaccordance with an embodiment of the disclosure. HMD 900 includes frame914 coupled to arms 911A and 911B. Lenses 921A and 921B are mounted toframe 914. Lenses 921 may be prescription lenses matched to a particularwearer of HMD or non-prescription lenses. The illustrated HMD 900 isconfigured to be worn on or about a head of a user of the HMD.

In FIG. 9, each lens 921 includes a waveguide 950 to direct image lightgenerated by a display 930 to an eyebox area for viewing by a wearer ofHMD 900. Display 930 may include an LCD, an organic light emitting diode(OLED) display, micro-LED display, quantum dot display, pico-projector,or liquid crystal on silicon (LCOS) display for directing image light toa wearer of HMD 900.

The frame 914 and arms 911 of the HMD 900 may include supportinghardware of HMD 900. HMD 900 may include any of processing logic, wiredand/or wireless data interface for sending and receiving data, graphicprocessors, and one or more memories for storing data andcomputer-executable instructions. In one embodiment, HMD 900 may beconfigured to receive wired power. In one embodiment, HMD 900 isconfigured to be powered by one or more batteries. In one embodiment,HMD 900 may be configured to receive wired data including video data viaa wired communication channel. In one embodiment, HMD 900 is configuredto receive wireless data including video data via a wirelesscommunication channel.

Lenses 921 may appear transparent to a user to facilitate augmentedreality or mixed reality where a user can view scene light from theenvironment around her while also receiving image light directed to hereye(s) by waveguide(s) 950. Consequently, lenses 921 may be considered(or include) an optical combiner. In some embodiments, image light isonly directed into one eye of the wearer of HMD 900. In an embodiment,both displays 930A and 930B are included to direct image light intowaveguides 950A and 950B, respectively.

The example HMD 900 of FIG. 9 includes an array of infrared emitters(e.g. infrared LEDs) 960 disposed around a periphery of lens 921B inframe 914. The infrared emitters emit light in an eyeward direction toilluminate an eye of a wearer of HMD 900 with infrared light. In oneembodiment, the infrared light is centered around 850 nm. Infrared lightfrom other sources may illuminate the eye as well. The infrared lightmay reflect off the eye and be received by a Fresnel reflector 999selectively coated with a hot mirror and configured to direct and focusthe reflected infrared light to camera 947. Fresnel reflector 999 mayhave an off-axis Fresnel lensing shape to direct the reflected infraredlight to camera 947. In this way, camera 947 is able to image the eye ofa wearer of HMD 900. Camera 947 may be mounted on the inside of thetemple of HMD 900. The images of the eye captured by camera 947 may beused for eye-tracking purposes. The optical structures disclosed inFIGS. 4A-8E may be utilized as Fresnel reflector 999. Although theoptical structure may be in the user's vision, the Fresnel surfacepasses scene light to the eye essentially unaffected by the Fresnelsurface, as discussed in connection with FIGS. 4-8E. The Fresnelreflector 999 can be included in lenses 921 as covering the wholewaveguide 950 or covering a portion of waveguide(s) 950. Although camera947, infrared emitters 960, and Fresnel reflector 999 are illustrated ononly one side of HMD 900, they of course may be duplicated on the otherside of HMD 900 to facilitate infrared imaging of both eyes of a wearerof HMD 900.

The term “processing logic” in this disclosure may include one or moreprocessors, microprocessors, multi-core processors, Application-specificintegrated circuits (ASIC), and/or Field Programmable Gate Arrays(FPGAs) to execute operations disclosed herein. In some embodiments,memories (not illustrated) are integrated into the processing logic tostore instructions to execute operations and/or store data. Processinglogic may also include analog or digital circuitry to perform theoperations in accordance with embodiments of the disclosure.

A “memory” or “memories” described in this disclosure may include one ormore volatile or non-volatile memory architectures. The “memory” or“memories” may be removable and non-removable media implemented in anymethod or technology for storage of information such ascomputer-readable instructions, data structures, program modules, orother data. Example memory technologies may include RAM, ROM, EEPROM,flash memory, CD-ROM, digital versatile disks (DVD), high-definitionmultimedia/data storage disks, or other optical storage, magneticcassettes, magnetic tape, magnetic disk storage or other magneticstorage devices, or any other non-transmission medium that can be usedto store information for access by a computing device.

A computing device may include a desktop computer, a laptop computer, atablet, a phablet, a smartphone, a feature phone, a server computer, orotherwise. A server computer may be located remotely in a data center orbe stored locally.

Embodiments of the invention may include or be implemented inconjunction with an artificial reality system. Artificial reality is aform of reality that has been adjusted in some manner beforepresentation to a user, which may include, e.g., a virtual reality (VR),an augmented reality (AR), a mixed reality (MR), a hybrid reality, orsome combination and/or derivatives thereof. Artificial reality contentmay include completely generated content or generated content combinedwith captured (e.g., real-world) content. The artificial reality contentmay include video, audio, haptic feedback, or some combination thereof,and any of which may be presented in a single channel or in multiplechannels (such as stereo video that produces a three-dimensional effectto the viewer). Additionally, in some embodiments, artificial realitymay also be associated with applications, products, accessories,services, or some combination thereof, that are used to, e.g., createcontent in an artificial reality and/or are otherwise used in (e.g.,perform activities in) an artificial reality. The artificial realitysystem that provides the artificial reality content may be implementedon various platforms, including a head-mounted display (HMD) connectedto a host computer system, a standalone HMD, a mobile device orcomputing system, or any other hardware platform capable of providingartificial reality content to one or more viewers.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An optical structure comprising: a firstrefractive layer having a first refractive index; a Fresnel surfaceformed in a second refractive layer having a second refractive index,wherein the Fresnel surface includes active surfaces and draft surfaces;a dichroic reflective coating selectively disposed on the activesurfaces; a third refractive layer; and a replica layer disposed betweenthe Fresnel surface and the third refractive layer, the replica layerhaving the second refractive index, wherein the second refractive indexis higher than the first refractive index.
 2. The optical structure ofclaim 1, wherein an average refractive index of the dichroic reflectivecoating is substantially the same as the second refractive index.
 3. Theoptical structure of claim 1 further comprising: a bonding materialdisposed between the dichroic reflective coating on the active surfacesand the replica layer, wherein the bonding material is also disposedbetween the draft surfaces and the replica layer, the bonding materialhaving the second refractive index.
 4. The optical structure of claim 1,wherein the replica layer is cured and adhered to the draft surfaces,the dichroic reflective coating, and the third refractive layer.
 5. Theoptical structure of claim 1, wherein the dichroic reflective coating isconfigured to pass visible light and reflect near-infrared light.
 6. Theoptical structure of claim 5, wherein the dichroic reflective coatingincludes at least a first layer, a second layer, and a third layer. 7.The optical structure of claim 1, wherein the first refractive layerincludes at least one of poly-methyl-methacrylate (PMMA), polycarbonate,cyclic-olefin polymer, styrene, or polyester.
 8. The optical structureof claim 1, wherein the third refractive layer has the first refractiveindex.
 9. The optical structure of claim 1, wherein the third refractivelayer is bonded to the replica layer.
 10. The optical structure of claim1, wherein the replica layer is a negative of the Fresnel surface.
 11. Ahead mounted display (HMD) comprising: a camera; and an optical combinerconfigured to pass visible light to an eyebox area and configured todirect infrared light to the camera, wherein the infrared light isreceived from the eyebox area, the optical combiner including: a firstrefractive layer having a first refractive index; a Fresnel surfaceformed in a second refractive layer having a second refractive index,wherein the Fresnel surface includes active surfaces and draft surfaces;a dichroic reflective coating selectively disposed on the activesurfaces; a third refractive layer; and a replica layer disposed betweenthe Fresnel surface and the third refractive layer, the replica layerhaving the second refractive index, wherein the second refractive indexis higher than the first refractive index.
 12. The HMD of claim 11,wherein an average refractive index of the dichroic reflective coatingis substantially the same as the second refractive index.
 13. The HMD ofclaim 11, wherein the replica layer is cured and adhered to the draftsurfaces, the dichroic reflective coating, and the third refractivelayer.
 14. The HMD of claim 11, wherein the dichroic reflective coatingis configured to pass the visible light and reflect the infrared light,and wherein the dichroic reflective coating is a multilayer coating. 15.A method of fabricating an optical structure, the method comprising:providing a first refractive layer having a first refractive index;forming a Fresnel surface in a second refractive layer, wherein theFresnel surface includes active surfaces and draft surfaces, and whereinthe second refractive layer has a second refractive index different thanthe first refractive index; selectively forming a coating on the activesurfaces; and forming a replica layer over the Fresnel surface, whereinthe replica layer has the second refractive index, and wherein thereplica layer is a negative of the Fresnel surface, the secondrefractive layer disposed between the first refractive layer and thereplica layer.
 16. The method of claim 15, wherein forming the replicalayer over the Fresnel surface includes bonding the replica layer to thecoating on the active surfaces with a bonding material having the secondrefractive index.
 17. The method of claim 16, wherein the replica layeris bonded to a third refractive layer having the first refractive index.18. The method of claim 16, wherein forming the replica layer over theFresnel surface includes: securing a third refractive layer over theFresnel surface; filling replica gaps between the third refractive layerand the Fresnel surface with a curable material having the secondrefractive index; and curing the curable material.
 19. The method ofclaim 18, wherein the third refractive layer has the first refractiveindex.
 20. The method of claim 18, wherein the curable material isadhered to the draft surfaces, the coating, and the third refractivelayer.